This seemed like a great deal to the gf and I when we were looking for pay as you go phones last week for her son but the main problem was we had to buy a nice phone and then pay to get it unlocked as well, adding to the costs, as T-mobile were quite expensive with there handsets.
We wanted him to remain with T-mobile because they seem to offer the best package for his particular needs and he gets to keep his number, so then we found the Sony Ericsson K750i on T-mobile, Vodaphone and Orange offering it for the same price.
You basically buy the phone for £69.95 as long as you also buy £10 airtime on the respective network and if you go through Quidco, you'll also receive £10 cashback, which means after the credit, which he'll use anyway and Quidco, the phone will work out at under £60 for a good PAYG for a kid or an adult for that matter.
I'm not sure if the deal is also available on the highstreet but you obviously won't receive £10 from Quidco, if that's the option you choose but there's always every possability you could negotiate further to make up the difference. :)
Hope that helps anyone looking for a good phone at a low price. :)